In this paper we present a comparison of the results of FC magnetizati
on measurements on several Pb-Sr-(Y,Ca)-Cu-O crystals representing var
ious levels of flux pinning. The pinning centers in our crystals have
been set up during the crystal growth process or introduced by neutron
irradiation. Some possible explanations of the observed effects, incl
uding surface barrier, flux-center distribution and sample-shape effec
ts, are discussed.